Rusts on plants (Puccinia)

Rusts are one of the most complex plant pathogens, with a diverse range of species estimated in size somewhere between 5000-6000, as well as a large range of hosts. Hosts include dicotyledonous families such as Compositaceae, Euphorbiaceae, Leguminaceae and Moncot families such as Orchidaceae and Lilaceae, and ofcourse of most international importance Poaceae (grasses, mayz & Wheat).

Rusts could be described as visible black, yellow, orange, red, or brown pustules growing on the plant. Initially starting out as pale leaf spots. The majority of Rusts are found on leaves, but they may also be found on flowers, fruits or stems. Puccinia (Rust) requires two separate hosts throughout its life cycle, making it heteroecious.

Rust cycles through 5 different spore types. These are known as Aeciospores, Spermatia, Urendiniospores, Teliospores, and Basidiospores. Basidiospores infect the alternate host and germinate creating a haploid mycelium. Spermatia and receptive hyphae create Aeciospore pustules which then release Aeciospores. Aeciospores infect plants through their stomata. Urendiospores are then produced, eventually creating visible teliospore structures (pustules) at the end of Summer. Puccinia then overwinter as black diploid teliospores, releasing basidiospores in the spring.

Fungicide products can be employed to control Rust, but best practice is the changing of growing circumstances through clever cultivar choice and environmental manipulation to create unfavourable growing conditions. Organic controls for gardeners include Neem oil, Sulphur, or baking soda

Gene editing has been frowned upon over the last decade for obvious reasons, but Scientists in China have had success with gene editing on wheat crops using the infamous and controversial CRISPR method (CRISPR BABIES). Instead of using Rust resistant genes from other crop types, which was the initial theory, the method looks to improve the performance of the already existent Rust resistant genes in Wheat.

Image: Rust on Hollyhocks : Hollyhock Rust | Center for Agriculture, Food, and the Environment (umass.edu)

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